The drying of large quantities of particulate or powdery materials to a desired moisture content provides difficulties especially for heat sensitive materials such as grain. Without uniform drying, degradation of heat sensitive materials may occur in the hot areas of the dryer, while other areas of the dryer may not sufficiently dry the particulate material without prolonged drying times. There is a balance therefore between the drying time and the drying temperature that is used to dry the particulate material that must be met, particularly if the particulate material is prone to degradation from overheating. For example, if grain is heated too quickly and then quickly cooled during the drying operation, the sudden temperature changes may tend to cause stress cracking and shattering of the grain. Such cracking or shattering of the grain greatly lowers the value of the grain such that it may not be acceptable to many grain elevators and processors.
In some instances, uniformity of the moisture content of the dried particulate material is ultimately important. Excessive moisture in a portion of the particulate material after drying may present problems with handling, particularly if the material is prone to agglomeration in the presence of moisture. Furthermore, the presence of undesirable moisture may increase the corrosion rate of storage vessels containing particulate materials such as halogenated catalysts and the like. In the foregoing examples, uniform drying of particulate material to a desired moisture level is an important consideration.
Many systems have been developed over the years which are intended to heat and uniformly dry particulate material such as grain while at the same time avoiding problems associated with drying heat sensitive materials. One such system is the cross-flow column type particulate dryer in which heated air is transversely forced through a moving bed of particulate material so as to dry the material. Because the heated air is hotter on one side of the drying column than on the other, difficulties are encountered in trying to provide uniform drying across the bed of particulate material in the dryer. Particulate material closest to the hot air inlet will generally be dried faster and to a greater degree than material on the opposite side of the dryer from the hot air inlet.
Counter flow drying systems may also be used to dry particulate materials. In this case, heated air is forced through the particulate material in a direction opposite to the direction of flow of the material through the dryer. As with the cross-flow dryers, the material nearest the hot air inlet of the counter flow dryer is hotter and is dried more than the material near the air exhaust outlet. In order to obtain a desired moisture content for the particulate material as a whole, a portion of the material may have to be over dried or otherwise overheated. While counter flow dryers may be more efficient, they are not particularly well suited to the drying of heat sensitive materials.
A particularly useful dryer for heat sensitive materials is the concurrent-countercurrent flow dryer in which heated air travels through the particulate material in the same direction as the moving bed of particulate material and a countercurrent flow of cooling air travels in a direction opposite to the direction of flow of the particulate material. In this system, air exhaust means are provided intermediate the hot air inlet and cooling air inlet of the dryer. With such a system, the hottest air is available at a point in the dryer whereby it is of the greatest value, i.e. wherein the particulate material is the wettest and coolest. As the particulate material and hot air travel together, the air heats and dries the particulate material while the moisture removed from the particulate material gradually cools the air. The counter flow of cooling air serves to further cool and temper the particulate material before it reaches the outlet of the dryer.
One of the earliest concurrent-countercurrent dryers is described in U.S. Pat. No. 2,706,343 to Oholm. However, the Oholm system has the disadvantage of presenting a V-shaped particulate surface area exposed to the entering hot air. This V-shaped surface area of the particulate bed typically results in the particulate material falling free from a feed spout into a pile having sloped conical sides. Thus, the length of time that individual particles are exposed directly to the hot air will vary according to their position on the pile, with those particles near the peak of the pile being exposed for a shorter period than those on the side slopes of the pile. With such a system, in order to arrive at some acceptable average moisture level for the bulk of the particulate material flowing through the dryer, it may be necessary to over dry at least a portion of the particulate material.
Many of the above problems associated with particulate dryers have been solved by the dryer disclosed in U.S. Pat. No. 4,086,708 to Westelaken. Westelaken describes an improved hot air contacting arrangement for a dryer which includes a wet material bin having a horizontal floor assembly with a plurality of uniformly spaced openings with a tube member extending downwardly beneath each such opening. These tubes serve to deliver wet particulate material in response to gravity from the wet material bin into a drying zone. The floor assembly permits the wet particulate material to be cyclically delivered into the drying zone with a pulsating action. This pulsating action causes a lateral flow of particulate materials resulting in layers of wet particulate material being deposited in the drying zone for contact with hot air. A hot air inlet duct is provided adjacent the tube members to deliver hot air into the spaces between the tube members and downwardly through the bed of particulate material in the drying zone. Below the drying zone is a cooling zone. The cooling zone has metering outlets for the particulate material as well as cooling air inlet ducts. Exhaust air ducts are provided intermediate the drying and cooling zones.
In the Westelaken system, the efficiency of the dryer is based, at least in part, on the use of drying temperatures in excess of 500.degree. F. (260.degree. C.) near the hot air inlet. Accordingly, careful control over the flow of particulate material and hot air are essential to obtain a uniform rate of particulate drying across the drying zone. Any blockages in the flow of the particulate material or channeling of the air in the bed of particulate material must be avoided, particularly for heat sensitive materials.
In concurrent-countercurrent flow dryers, the burner size for providing heat and the tower size are all generally determined by volume and moisture content of the particulate material to be dried. Accordingly, a system designed to dry very wet material may require very high heat inputs and very high air flow volumes. Such concurrent-countercurrent dryers typically operate at their greatest efficiency when the air flow within the dryer is maintained at the minimum necessary to remove the desired amount of moisture from the particulate material, while still maintaining uniform air distribution. However, the moisture level of some particulate material, such as grain, is dependent on weather conditions in the region where the grain is grown and stored. In a region with traditionally high moisture levels, there may be a year of low moisture and hence very low particulate material moisture levels. Thus, there may be a need to adjust the air distribution within the dryer to account for variations in the moisture content of the particulate materials. There may also be a need to adjust the air distribution if the same dryer is used for a variety of particulate material, each material having its own drying characteristics.
It is an object of the invention to provide a dryer suitable for the uniform drying of large quantities of particulate materials.
It is another object of the invention to provide a concurrent-countercurrent dryer having a modified air discharge duct for essentially even distribution of drying and cooling air.
Yet another object of the invention is to provide an adjustable air exhaust mechanism for a particulate dryer.
Still another object of the invention is to provide a concurrent-countercurrent dryer for the uniform drying of particulate material wherein the drying air flow volume and distribution can be readily adjusted for variations in the moisture content of the particulate material to be dried.
Other objects and benefits of the invention will be evident from the following discussion and appended claims.